This invention relates to filters for the treatment of radioactive fluids generally used in the process systems of a nuclear power plant. More particularly, the filters of the present invention are of the type which include a casing with the filtering element mounted on the inside. The casing includes a cover and is housed inside a radiation shielding chamber. The chamber is provided with access openings which can be closed by means of radiation shielding gates or slabs. The filter is positioned behind one of the openings to provide access to the filter.
The filters used in the liquid process systems of nuclear power plants or radioactive installations are subjected to high levels of radioactivity. It is generally recognized that the main function of these filters is to retain radioactive corrosion products.
The problem which generally arises with filters which retain radioactive particles is not the loss of its filtering characteristics, but rather the build-up of the level of radioactivity. This build-up limits the operational conditions and affects the manner by which the filtering elements are changed, handled and disposed of. Experience has shown that costs associated with handling the spent filter elements are between ten and fifteen times, the cost of the filter element being changed.
Filters used in processing radioactive liquids in nuclear power plants or other installations trap high concentrations of radioactive corrosion particles or products on the inside, which give rise to a very high radiological dose rate in its vicinity. This means that it is necessary to handle the filters in special cells or rooms designated with adequate protection against radiations. Such rooms generally are constructed with walls, floor and roof made of materials of an adequate type and thickness to form a biological shielding against such radiation.
As earlier indicated, it is the high radiological dose rate which limits the operating time of the filtering elements in the majority of cases rather than the exhaustion of the filtering properties. The great increase in the radiological dose creates serious maintenance problems in the habitual filter installations, particularly during the element renewal operations.
Accordingly, in the design of a filter, or a bank of filters for a nuclear power plant or similar installation, factors such as the size of the filtering elements, the number of times it must be renewed per year, the permissible activity level, element renewal procedure, etc., must be taken into account.
In general, the appropriate design criteria for handling the filters are as follows:
1. During all the phases of filter element changing, the design must be capable of yielding a sufficiently low radiation level so as not to create problems for the installing operating personnel. The design shall be such that by following the operational sequences, the personnel shall not be exposed to level doses greater than the installation operational level doses, in accordance with the classification of the zones next to the filters. PA1 2. The system must be capable of handling filtering elements with relatively high dose level, in the region of 500 R/hr, when in contact with the filter casing. PA1 3. In principle, it must be guaranteed that there shall not be any need to impose special personnel limitations during the period when filtering elements are changed.
As previously mentioned, due to the high level of radiation which can be reached on the outside surface of the filters which treat radioactive liquids, the normal practice is to place the filters inside cells or cubicles enclosed by walls, floors and roofs of adequate materials and thicknesses to reduce the dose rates to acceptable values. These cells or cubicles are provided with access opening with closure slabs with at least one of these openings in front of the filter to provide access to the same. The conventional method for changing the cartridge, or filtering element requires removing the shielding slab of the cell or cubicle, after which the filter cover is removed. Once the slab or door of the cell or cubicle is removed, the radiation level is high, which means that the filter cover opening operation requires special remote control tools and/or additional shielding plates. Thus, the extraction of the filtering element from its environment requires a series of complex operations. However, once the shielding is removed there is a high radiation level which must be taken into account. After the filtering element is removed from the casing, it is usually placed inside a shielded cask for transportation. In the interim it is necessary to place a shielded cover or slab over the access opening of the same in order to close it. All these operations have to be carried out under a high level of radiation.